An integrated-services network, as known, employs the same transmitting and switching units for a wide range of services (telephone, video, data transmission etc.) with quite different characteristics such as:
transmission rates (and consequently band occupation) widely differing from one another, and possibly variable (e.g. from some kbit/s to many Mbit/s); PA0 circuit or packet switching depending on whether or not the information flow presents synchronous characteristics; PA0 maximum accepted delay ranging from few milliseconds to some hours.
In these systems it has proved advantageous to subdivide the network activity into repetitive frames, each composed of two regions respectively assigned to circuit-switched communications (basically time-division multiplexed, so that each call is allotted a time slot or channel of duration proportional to transmission rate) and to packet-switched communications. These frames are generally referred to as hybrid frames.
With such an organization of the information flow, the switching nodes in the network ought to have the flexibility necessary to process each communication according to its characteristics, without imposing restraints on transmission modalities and while reducing to a minimum any format changes in the information flow.
Such nodes are generally called "asynchronous nodes".
A number of realizations of these asynchronous nodes are known in the art.
A first solution is described by M. J. Ross, A. C. Tabbot and J. A. Waite in the paper titled "Design Approaches and Performance Criteria for Integrated Voice/Data Switching" (Proceedings of the IEEE, Vol. 65, No. 9, September 1977, pages 1283 ff.) and by M. J. Ross, J. H. Gottschalk and E. A. Harrington in a paper titled "An architecture for a flexible integrated voice/data switch", presented at the International Conference on Communication, Seattle, 8-12 June 1980.
According to this system a node comprises a plurality of modules connected on one side to subscribers or trunks conveying hybrid frames and on the other side to a pair of common buses for signalization and for data, respectively; a single nodal module, also connected to the two buses, having routing and supervising functions; and a timing module. For each communication the information arriving from subscribers or trunks is stored inside the respective module, basically at the instant at which it arrives; such information is presented on the bus upon request of the addressee module in a time phase in which that module can access the bus.
Hence the operation inside the node is synchronous, as each module is assigned a predetermined time slot. A solution of this kind simplifies the information-transfer procedures but presents a number of inconveniences; on account of the fixed-duration intervals for information reading, the node satisfactorily processes circuit-switched communications but is scarcely efficient for wide-band or packet-switched communications inasmuch as information-flow splitting may be required. The single timing module which is to carry out the most important system functions operates as a reliability node. A further limitation on the efficiency of the node is that an interval assigned to one module cannot be used by other modules even if that module has no operation to carry out; node-capacity expansion can require a modification of the nodal and timing modules.
A second solution has been described by M. Devault, J. P. Quinquis and Y. Ronaud in a paper titled "Asynchronous time division switching: a new concept for ISDN nodes", presented at ISS'81 (Montreal, 21-25 September 1981).
According to this latter solution a node consists of a plurality of modules coupled to local lines or trunks and interconnected by a set of buses allowing complete and direct accessibility from one module to any other. Inside the node the information relevant to the input and output channels (time solts) is associated with each sample of a circuit-switched communication or with each packet incoming to the node, and blocks are formed which are asynchronously switched by following substantially a packet-switching procedure.
Thus, problems relevant to the presence of centralized modules are avoided, and variable-band communications can be processed; however, signal processing is needed also for circuit-switched communications, which moreover require reduction of the variability of transit delay through the network since such delay, as known, can degrade communication quality.
The use of hybrid frames for information exchange within a communication system is described for instance in commonly owned U.S. application Ser. No. 502,084 filed by me jointly with another on June 8, 1983. Tha prior application, however, concerns the information exchange among different nodes of a switching system and not inside a node; besides, it requires the frame-by-frame memorization of the activity on the bus and therefore does not allow dynamic management of hybrid frames with variation in the band assigned to each synchronous communication and with frame-by-frame channel recompaction.